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1 CELLULAR RESPIRATION 1

2 The Pathway of energy in living organisms photosynthesis Light energy from the sun Chemical energy stored in glucose, fats, or carbohydrates cellular respiration Chemical energy for use in the form of ATP All living organisms must perform cellular respiration (plants and animals) to get ATP. Only photosynthetic organisms like plants or blue-green alga can transform light energy into chemical energy 2

12 There are four sections of critical importance in the pathway: 1-glycolysis, an enzymatic path in the cytosol 2-tricarboxylic acid cycle, an enzymatic path in the mitochondrial matrix 3-electron transfer system, an electronic path in the mitochondrial inner membrane 4-oxidative phosphorylation, a proton driven ATP synthesis between the intermembrane space and the matrix of the mitochondrion 12

13 Cellular Respiration Breakdown of glucose begins in the cytoplasm: the liquid matrix inside the cell At this point life diverges into two forms and two pathways Anaerobic cellular respiration ( fermentation) Aerobic cellular respiration 13

14 Celluar Respiration Reactions Glycolysis Series of reactions which break the 6-carbon glucosemolecule down into two 3-carbon molecules called pyruvate All organisms from simple bacteria to humans perform it the same way Yields 2 ATPmolecules for every one glucose molecule broken down Yields 2 NADHper glucose molecule 14

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16 In this process (glycolysis), glucose undergoes partial oxidation to form two molecules of pyruvic acid. In plants, this glucose is derived from sucrose, which is the end product of photosynthesis, or from storage carbohydrates. Sucrose is converted into glucose and fructose by the enzyme, invertase, and these two monosaccharides readily enter the glycolytic pathway. Glucose and fructose are phosphorylated to give rise to glucose-6- phosphate and fructose-6-p respectively by the activity of the enzyme hexokinase. The phosphorylated form of glucose then isomerises to produce fructose-6-phosphate. Subsequent steps of metabolism of glucose and fructose are same. 16

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20 In glycolysis, a chain of ten reactions, under the control of different enzymes, takes place to produce pyruvate from glucose. Utilisation of ATP During Glycolysis: 1. During the conversion of glucose into glucose 6-phosphate 2. During the conversion of fructose 6-phosphate to fructose 1, 6-bisphosphate. There are three major ways in which different cells handle pyruvic acid produced by glycolysis. These are lactic acid fermentation, alcoholic fermentation and aerobic respiration. Fermentation takes place under anaerobic conditions in many prokaryotes and unicellular eukaryotes. For the complete oxidation of glucose to CO 2 and H 2 O, however, organisms adopt Krebs cycle which is also called as aerobic respiration. This requires O 2 supply. 20

21 Anaerobic Cellular Respiration Some organisms thrive in environments with little or no oxygen No oxygen used= an aerobic Results in no more ATP, final steps in these pathways serve ONLY to regenerate NAD+ so it can return to pick up more electrons and hydrogens in glycolysis. End products such as ethanol and CO 2 (single cell fungi, yeast) or (lactic acid in humans muscles) Sugars are the most common substrate of fermentation, and typical examples of fermentation products are ethanol, lactic acid, and hydrogen 21

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23 Aerobic Cellular Respiration Pyruvate, which is formed by the glycolytic catabolism of carbohydrates in the cytosol, after it enters mitochondrial matrix undergoes oxidative decarboxylation to acetyl CoA by a complex set of reactions catalysed by pyruvic dehydrogenase. The reactions catalysed by pyruvic dehydrogenase require the participation of several coenzymes, including NAD + and Coenzyme A. It takes place in the matrix of the mitochondria. During this process, two molecules of NADH are produced from the metabolism of two molecules of pyruvic acid (produced from one glucose molecule during glycolysis). 23

25 Kreb s Cycle Completes the breakdown of glucose Takes the pyruvate(3-carbons) and breaks it down, the carbon and oxygen atoms end up in CO 2 and H 2 O Hydrogensand electrons are stripped and loaded onto NAD + and FAD to produce NADH and FADH2 Production of only 2 more ATP but loads up the coenzymes with H + and electrons which move to the 3 rd stage 25

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28 The acetyl CoA then enters a cyclic pathway, tricarboxylic acid cycle, more commonly called as Krebs cycle after the scientist Hans Krebs who first explained it. 28

29 Electron Transport Chain Electron carriers loaded with electrons and protons from the Kreb scycle move to this chain-like a series of steps (staircase). As electrons drop down stairs, energy released to form a total of 34 ATP Oxygenwaits at bottom of staircase, picks up electrons and protons and in doing so becomeswater 29

40 But what pulls the electrons down the ETC? electrons flow downhill to O 2 40

41 Electrons from NADH produced in the mitochondrial matrix during citric acid cycle are oxidized by an NADH dehydrogenase (complex I), and electrons are then transferred to ubiquinone (coenzyme Q) located within the inner membrane. Ubiquinone also receives reducing equivalents via FADH 2 (complex II) that is generated during oxidation of succinate in the citric acid cycle. 41

42 The reduced ubiquinone(ubiquinol) is then oxidisedwith the transfer of electrons to cytochromecvia cytochromebc 1 complex (complex III). Cytochromec is a small protein attached to the outer surface of the inner membrane and acts as a mobile carrier for transfer of electrons between complex III and IV. Complex IV refers to cytochrome c oxidase complex containing cytochromesa and a 3, and two copper ions. 42

49 When the electrons pass from one carrier to another via complex I to IV in the electron transport chain, they are coupled to ATP synthase(complex V) for the production of ATP from ADP and inorganic phosphate. The number of ATP molecules synthesized depends on the nature of the electron donor. Oxidationof onemolecule of NADH gives rise to 3 molecules of ATP,while that of one molecule of FADH 2 produces 2 molecules of ATP. Although the aerobic process of respiration takes place only in the presence of oxygen, the role of oxygen is limited to the terminal stage of the process. Yet, the presence of oxygenis vital, since it drives the whole process by removing hydrogen from the system. Oxygen acts as the final hydrogen acceptor. 49

50 The energy released during the electron transport system is utilized in synthesizing ATP with the help of ATP synthase (complex V). This complex consists of two major components, F 1 and F 0. The F 1 headpiece is a peripheral membrane protein complex and contains the site for synthesis of ATP from ADP and inorganic phosphate. F 0 is an integral membrane protein complex that forms the channel through which protons cross the inner membrane 50

51 The passage of protons through the channel is coupled to the catalytic site of the F 1 component for the production of ATP. For each ATP produced, 3 H + passes through F 0 from the intermembrane space to the matrix down the electrochemical proton gradient 51

56 Unlike photophosphorylation where it is the light energy that is utilized for the production of proton gradient required for phosphorylation, in respiration it is the energy of oxidationreduction utilized for the same process. It is for this reason that the process is called oxidative phosphorylation 56

58 Summary of ATP production Stage 1 and 2 release all the chemical energy in one molecule of glucose to make a total of 38 ATP molecules. 2 molecules ATP from glucose pyruvic acid 36 molecules ATP from pyruvic acid carbondioxide + water Total 38 molecules ATP 58

59 anaerobic = in the absence of oxygen ANAEROBIC RESPIRATION IN PLANTS The same process occurs in plants and yeast in low oxygen conditions 59

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